An OLED can be formed so that it is down emitting and the invention may be particularly useful for such down emitting OLEDs. Down emitting means that the OLED emits light through the substrate on which the OLED is formed and is viewed through the substrate on which it is formed.
With reference to FIG. 1, a simplified structure over which a down emitting OLED 10 is built, is shown. The OLED 10 may include a transparent substrate 100 on which plural hole injector lines 200 are formed. The hole injector lines 200 may typically comprise indium tin oxide (ITO). Substantially perpendicular to, and overlying the hole injector lines 200 may be columns of organic light emitting material 300. The organic light emitting material may be referred to as an organic stack because a stack of different organic materials is often required in order for the overall stack to emit light of a desired wavelength. Electron injector columns 400 (e.g. Mg/Ag) may overlie the organic stack columns 300.
A pixel may be formed at each portion of the organic stack that is sandwiched between a section of a hole injector line 200 and a section of an electron injector column 400. Light may be generated in a pixel when the appropriate voltages are simultaneously applied to the respective hole injector line 200 and electron injector column 400 that lie below and above the pixel. The generated light is emitted downward through the substrate 100 because the electron injector columns 400 are provided with a reflective coating, and the hole injector lines 200 and substrate 100 are transparent.
In order for the OLED 10 to operate properly, the electron injector columns 400 must be electrically isolated from each other. The process for making down emitting OLEDs may often end with the formation of electron injector columns 400 on top of the device 10. If the electron injector columns 400 are not appropriately isolated from each other, there is opportunity for cross-talk and electrical shorts between the electron injector columns.
With reference to FIG. 2, in which like reference numerals refer to like elements, one method of reducing the likelihood of cross-talk and electrical shorts between the electron injector columns 400 is to provide isolation columns 500 between each of the electron injector columns. The isolation columns 500 may provide a physical barrier of non-conductive material between the electron injector columns 400. The presence of the isolation columns 500 makes it more difficult for a short or cross-talk to occur.
A particular OLED design with isolation columns may be described with reference to the device 10 shown in FIG. 3. The device 10 may include isolation columns 500 comprising a bottom inorganic SiO layer 510 (which may alternatively include dielectric materials, e.g., Si.sub.3 N.sub.4, Al.sub.2 O.sub.3,AlN, or diamond like carbon DLC), an intermediate inorganic SiO.sub.2 layer 520, and a top inorganic SiO layer 530.
The device 10 shown in FIG. 3 may be formed by providing a first photoresist mask on the top of the device prior to the formation of the isolation columns 500. The first mask may be in the form of parallel spaced columns of photoresist material. The bottom SiO layer 510 may be formed be applying a blanket coating of SiO over the first mask and then removing the mask along with the SiO formed on top of the mask. After the formation of the bottom inorganic SiO layer 510, blanket coatings of inorganic SiO.sub.2 or other materials that can be differentially etched may be applied. Following the application of SiO.sub.2, a second layer of SiO may be applied to the top of the device 10. A second photoresist mask pattern of columns may then be formed on top of the top blanket layer of SiO. The isolation columns 500 may be completed by a RIE process of the top SiO layer and intermediate SiO.sub.2 layers through openings in the second photoresist mask. Following the RIE process, the SiO.sub.2 layer may be further etched using Buffered Oxide Etch (BOE) to complete the removal of the SiO.sub.2 layer.
The device 10 may be completed by providing blanket coatings of organic stack material 300 and electron injector material to the upper surface of the device. The overhang of the top SiO layer 530 is required to provide a separation distance d between the electron injector material 402 in the channel 502 and the electron injector material 404 on top of the top SiO layer 530. Increasing distance d tends to decrease the likelihood of electrical shorts and cross-talk between the electron injectors in adjacent channels 502. Without separation distance d, the electron injector material 404 on top of the top SiO layer 530 could provide a circuit bridge between the electron injector material 402 and the electron injector material in the next channel to the left or right (not shown).
The device 10 shown in FIG. 3, as well as the associated method, provides excellent structural integrity (i.e. the device holds together well) and is free from any residual moisture. The existence of the electron injector material 404 on top of the top SiO layer 530 may provide enough of a conductive bridge to interfere with the isolation function of the isolation columns 500.
One way to enhance the isolation function of the isolation columns 500 shown in FIG. 3, may be to increase the overall thickness of the isolation column 500, thereby increasing separation distance d. Increasing distance d tends to reduce the likelihood of electrical shorts and cross-talks between adjacent electron injector columns that may be caused by particles or contamination on the surface of the device 10. However, the thickness of the intermediate SiO.sub.2 layer 520, which may account for a substantial portion of the overall thickness of the isolation column 500, is limited by several factors. One such limitation is throughput of the evaporation process and ultimately of the product, i.e. the thickness of the SiO.sub.2 layer is limited by the amount of SiO.sub.2 material that can be applied per evaporation run, or per unit time. It may be prohibitively time consuming and labor intensive to subject the device 10 to the number of evaporation runs that are required for an SiO.sub.2 layer the desired thickness for an effective isolation column. Another limitation is that of top patterned photoresist mask integrity during RIE processing. The photoresist mask may erode if the RIE process is prolonged, as is required when the SiO.sub.2 layer is made thicker.
Another way to enhance the isolation function of the isolation columns 500 may be to remove the electron injector material 404 on top of the top SiO layer 530. Removal of the electron injector material 404 may effectively increase the distance d between the electron injector material 402 disposed in adjacent channels 502.